Electrical brain activity or EEG is measured by means of EEG electrodes contacting the skin of a user, in particular the scalp of the user. One type of known electrodes are wet or gel electrodes. Another type of known electrodes are dry electrodes (not using gel). The main challenge when applying EEG wet or gel electrodes is to get a good, thus low, contact impedance to the skin. In clinical measurements this is normally done with a (shower-cap like) rubber cap with integrated metal electrodes (e.g. Ag/AgCl coated). The skin underneath these electrodes usually needs to be prepared by degreasing and often additional abrasion (e.g. removal of the dry top layer of the skin, the stratum corneum). The conductive gel is then applied between each electrode and the scalp, typically through a hole in the electrode or cap. This assures a low ohmic contact to the deeper skin layer, the epidermis, and “conversion” from ion current in the body to electron current in the measuring system. Using conductive gel also solves (partly) the problem of the varying distance between the electrode and the skin due to the variation from person to person with respect to the hair layer thickness and the amount of hair, as well as temporal changes of the distance that might occur due to head and/or body motion.
For a lifestyle consumer product as well as for disabled patients and remote monitoring purposes in clinical applications, it is not practical to use this kind of “wet” electrodes. There are attempts to realize dry electrodes that use pin-structured electrodes or similar ways to penetrate the hair and make a galvanic contact to the skin. A problem arises when this type of electrodes has to cope with thick and long hair. In practice, the solutions often result in poor contact to the skin at the scalp and insufficient signal quality. Further, the electrode-skin contact impedance might differ for different electrodes and the variation of the skin contact impedance over time can differ for each electrode, posing a serious threat for practical applications.
An important application for convenient EEG measurements is brain wave sensing technology, such as alpha neurofeedback. Neurofeedback (NF), in particular alpha neurofeedback, is a novel method which may find application areas both in consumer and professional healthcare products. Boxtel et al. “A novel self-guided approach to alpha activity training”, International Journal of Psychophysiology, 2011, discloses that neurofeedback induces a feeling of ease in a person without the person feeling the burden of responsibility for his own mental state. This is particularly relevant for a hospital setting, where the user or patient is put at ease in a very subtle way without requiring them to be aware of the effect of neurofeedback. This is important for a hospital setting as it means the patient is not burdened with the feeling of having to relax.
To measure alpha brain wave activity in a convenient way, dry electrodes are necessary. In standard EEG measurements with gel-electrodes, measurements are typically done in controlled conditions, where the experimenter or trained person applies the gel and positions the EEG system on a user's head, checks whether the skin-electrode contact impedance is in range (i.e. less than 10 kΩ), and if the signal looks as expected. In real-life situations where an expert is not available and where the user applies the (headset with) dry electrodes extensive pre-measurements of the signal quality before the actual measurement are not an option. Users are not in the position to perform a thorough check of the signal quality. However, a sufficient signal quality is crucial to prevent users adhering to the wrong neurofeedback.
As a solution to this problem, WO 2011/055291 A1 discloses a device for positioning dry, pin-structured electrodes on a user's scalp. The device features an elastic element to exert pressure on a plurality of electrodes towards the scalp, thereby improving the contact of the electrodes to the skin. One way to further improve the signal quality is to monitor and adjust the electrode-skin contact pressure, which in turn changes the electrode-skin contact impedance.